Macrophages are widely distributed, specialized phagocytic cells which play a major role in host defense against infection, in the course of which they may become activated and gain an enhanced capacity to kill micro-organisms and possibly tumor cells. Our recent studies show that macrophages are also secretory cells able to release a variety of potent enzymes into their extracellular environment. These include lysozyme, a cell-specific bulk product secreted continuously by all macrophages, and several specific neutral proteases including plasminogen activators, collagenase and elastase, inducible products secreted only in response to cell stimulation and phagocytosis. Future objectives are to characterize these and other secretion products biochemically, to identify their intracellular location and secretory pathway by cytologic methods, to study the regulation of their production and release in cell culture and to explore their possible role in macrophage effector functions in vitro and in vivo. Macrophage secretion brings about specific lysis of extracellular proteins like fibrin, collagen and elastin and facilitates repair after tissue injury. Plasmin can also activate enzymatic cascades like complement and generate mediators of inflammation. The continued secretion of proteases following phagocytosis of undegradeable particles could perpetuate chronic inflammation and promote significant tissue injury. Induction of secretion provides a biochemical marker for, and may contribute to, the bactericidal and tumoricidal capacity of the activated macrophage.